The present invention relates to a circuit arrangement for selectively reducing or compensating for the temperature non-linearity of the characteristics of piezoresistive measuring resistors which are connected in a bridge circuit.
It is known, for example, when measuring pressures, that micromechanical pressure sensors having a measuring membrane made of a piezoresistive material can be used. In order to obtain an output signal of the pressure sensor, it is known that the measuring membrane can be provided with piezoresistive resistors connected in a bridge circuit (Wheatstone bridge), through which a pressure applied to the measuring membrane can be converted into an electrical output signal.
The bridge resistors are composed of partial resistors which result from the selected design of the pressure sensor. The latter usually has a measuring membrane made of silicon, which is n-doped, for example, into which the actual measuring resistors are applied by diffusion, for example, as p-doped regions (base diffusion). Leads are provided for connecting the measuring resistors. The leads can be designed as a combination of low-resistance diffusion regions and metallic printed conductors. The diffused lead resistor can be formed by p+ doping, for example. The metallic lead resistor may be made of aluminum, for example. The bridge resistors thus each result from the sum of three partial resistors, namely the actual piezoresistive measuring resistor and the diffused and metallic lead resistors. Each of these partial resistors has a different non-linear temperature curve.
The circuit arrangement according to the present invention offers the advantage over the related art that the temperature non-linearity of the characteristic of a bridge circuit having piezoresistive measuring resistors can be selectively reduced or compensated in a simple manner. Due to the fact that the partial resistors of each bridge resistor are selected, i.e., dimensioned on the basis of their known, non-linear temperature characteristics, well-defined different characteristic curves can be attained for temperature and/or pressure (as a function of mechanical stresses), so that a non-linear temperature characteristic of the pressure transducer due to bimetal effects or non-linearities of the individual resistor elements or a temperature-dependent bridge base voltage, for example, can be compensated. In particular, the non-linear characteristic curve of the individual bridge resistors can be compensated by selecting the ratios of the resistances of the individual partial resistors within the total resistance of the bridge resistors, since in selecting the resistances, the known non-linear temperature and/or pressure response of the individual bridge resistors can be influenced. In particular, the optimum resistances can be determined individually for the partial resistors that are actually subject to a different effect, since the temperature response and/or pressure response have different effects on the individual partial resistors of the bridge resistor.
In a preferred embodiment of the present invention, the resistances of the partial resistors of the bridge resistors are selected so that within a bridge resistor partial resistors having a temperature characteristic with an inverted sign are taken into account. This allows non-linearities occurring as a result of a temperature effect to be compensated even within the bridge resistor, since the partial resistors have a corresponding positive or negative characteristic which are superimposed due to their integration in a bridge resistor.